Kilauea's
Volcanic
"Plumbing
System"

From years of monitoring and research at HVO, Kilauea's volcanic "plumbing
system" is now relatively well understood. This system links the processes
involved in the formation, transport, storage, and, ultimately, eruption of magma
to build and feed Hawaii's active volcanoes.

Kilauea's plumbing system is
believed to extend deep beneath the Earth's surface, where basaltic magma is
generated by partial melting of material beneath the Pacific Plate as it passes
over the Hawaiian hot spot. This belief is based on the persistent recurrence of
earthquakes 30 or more miles beneath Hawaii. Earthquakes occurring in the depth
interval 20-30 miles are probably related to the accumulation and upward movement
of magma. Seismic data for levels shallower than 20 miles can be interpreted to
define diffuse zones of continuous magma rise, one leading to Kilauea and another
to Mauna Loa.

Before Kilauea eruptions, most of the magma entering the volcano is
stored temporarily within a shallow reservoir. Earthquake data and
ground-deformation patterns suggest that this reservoir is located 1 to 4 miles
beneath the summit and consists of pockets of magma concentrated within a crudely
spherical volume about 3 miles across. Earthquakes do not occur within the
reservoir, because liquid magma does not rupture to generate and transmit certain
seismic waves.

Kilauea eruptions occur either at its summit or within two well-defined swaths
(called rift zones) that radiate from the summit. During summit eruptions, the
magma reservoir deflates only slightly, if at all. This relation implies that the
rate at which magma is erupted nearly equals that at which the reservoir is
refilled by new magma from depth. During an eruption in a rift zone, called a
rift or flank eruption, however, the summit region undergoes a significant and
abrupt deflation as magma moves quickly from the summit reservoir into the rift
zone. Similar summit deflation occurs during a rift intrusion, during which magma
injected into the rift zone remains stored there rather than breaking the ground
surface in an eruption. When the rift eruption or intrusion ends, the summit
region reinflates as the shallow reservoir is refilled by magma from depth. Small
pockets of summit-fed magma may be stored for a while within a rift zone and form
transient secondary reservoirs.

The volcanic plumbing system for Mauna Loa is
less well known. Analysis of data from the well monitored 1975 and 1984
eruptions, however, suggests that the essential features of Kilauea's plumbing
system are shared by Mauna Loa, despite the difference in size between the two
volcanoes. Mauna Loa's magma reservoir also may be larger than Kilauea's, which
would be consistent with the observations that Mauna Loa eruptions tend to be
characterized by higher lava-output rates, longer eruptive fissures, and larger
lava flows.